In the computing industry, laser printers have become the standard for producing high quality, hard copy results. Laser printers operate, generally, by scanning a laser beam axially (in the "scan direction") across a photosensitive, electrically charged drum or belt. A picture element or "pixel" is a basic building block for creating images. In a laser printer a pixel has the form of a "dot" which is essentially an electrical charge imprint produced by exposing a photoconductive drum or belt to a focused pulse of laser light. The exposed area attracts toner which is subsequently transferred to paper or other print medium to create a corresponding dot in the final hard copy image. In conventional laser printers, each time the laser beam is scanned across the photoconductive surface, the laser power is modulated or "pulsed" at a rapid rate, each pulse creating one dot and each scan pass creating one row of closely spaced dots (sometimes called a "scan line"). Simultaneously, the photoconductive surface is moving in the process direction (at right angles to the scan direction, and sometimes called the "cross-scan direction") so that each successive scan produces a row of dots parallel to the preceding row but displaced due to motion of the photoconductive surface, thus forming a two-dimensional image as many successive rows of dots are written. Image resolution, as defined by the number of separately resolvable dots per inch (dpi), is one of the characteristics that determine the quality of the printer output. For example, a 600 dpi resolution laser printer has twice the resolution of a 300 dpi printer. Similarly, a 1200 dpi printer has twice the resolution of a 600 dpi printer.
Generally, the more dots per inch the better the image quality because the dots are smaller and closer together and can thereby more accurately define the edges and other details of the images produced. However, an increase in dpi usually increases a printer's manufacturing cost due to the increased memory and improved hardware required to produce the increased dpi. Accordingly, balancing the trade-offs between cost and performance is an ongoing issue in laser printer design.
One conventional method of increasing a printer's resolution has been to pulse the laser at higher frequency as the beam moves across the photoconductive drum in a conventional linear scan path. However, this method increases resolution in the scan direction only, without increasing resolution in the process direction (circumferential relative to the drum). Namely, more dots per inch are achieved in the scan direction but not in the process direction.
The development of gray scale technology has provided an alternative method for effectively increasing resolution. Gray scale selectively reduces the dot size in a printer to provide a better "fit" of the dots to the desired image and to provide a greater contrast range. Dot size ranges from 100% to about 20% of normal size, and is controlled by varying the pulse width (duration of the laser modulation pulse), but not the frequency of modulation. This technique, called pulse width modulation (PWM), changes the local size of the dots but not their location. A 300 dpi printer which uses PWM therefore still produces a pattern of dots at 300 dpi spacing, although the individual dots vary in size.
In addition to resolution enhancement, another performance characteristic of laser printers that is a candidate for improvement is printing speed. Given the continuous technological improvements in computing speeds, printing speeds must likewise be improved to satisfy consumer demands. Generally, however, to increase printing speed in laser printers, the multi-sided scanning mirror (polygon scanner) and the photoconductive drum must both be rotated at faster rates to provide greater output per unit of time. However, attempts to increase drum and polygon scanner rotation speeds typically present numerous mechanical difficulties and result in significantly increased manufacturing costs for the printer.
If the laser pulse width is modulated during scanning, variations in charge on the photoconductive surface will be translated into variations in dot size on a sheet of paper (such as discussed with gray scale). However, since human vision is very sensitive to small changes in gray level, this architecture has proven to be extremely sensitive to variations in drum speed. These variations appear on the printed page as increased or decreased spacing between lines and visually appear as "bands" parallel to the scan direction, and this undesirable effect is called "banding". Banding is a particularly severe problem for faster laser printers which are printing gray scale images, such as photographs. Research has shown that the most severe banding effects occur at intermediate levels of gray.
Most commercial printers use stepper motors with geared speed reducers to drive the photoconductive drum or belt. The principal cause of banding is speed reducer gear noise, although speed variations in the stepper motor and scanner can also contribute to this problem. Gear noise results from imperfect spacing of gear teeth, variances in flexing of gear teeth as forces are transferred from one gear to the next, and other intrinsic variations in gear force transfer. The stepper motor contributes to banding as it drives the gear train in a laser printer because of slight variations in angular velocity due to irregularities in the multiple magnet positions which define each step.
Since new printer products are consistently designed to print faster, and since the causes of banding tend to worsen at higher speeds, the problem of banding is likely to worsen in the future. Conventionally, attempts to reduce banding effects have been focused on finding mechanical methods for reducing gear noise and stepper motor speed variations. For example, attempted mechanical solutions may involve using gears with helical teeth or gears made from stiffer materials or with greater precision, but these improvements generally add significantly to the cost of the final product. Furthermore, these approaches attempt to address the root cause of the banding problem by producing a mechanically ideal open loop (without position feedback) motion control system. Namely, they attempt to rotate the drum at a constant speed with a motor drive system in which no feedback from any source is used to modify the motor speed or to correct any of the previously described contributions to banding. Although the cited Lawton application discloses randomly varying laser beam scanning for enhancing edge resolution and reducing banding, its methods are limited with respect to improving printer speed and resolution.
Accordingly, given the foregoing backgrounds relating to laser printer resolution, printing speed, and banding, objects of the present invention are to provide a new system and method for (1) increasing resolution in a laser printer with minimal increase in the hardware costs conventionally associated with enhanced resolution printers, (2) increasing printing speeds, and (3) reducing the visual impact of banding.